Hydro is cheaper in the long run (no soil to buy) and reusuable, but initial start up cost is higher than soil growing. Check out the Hydro forum and check out their grows for yourself.
If you just have a cupboard, you probably want to grow with CFL's. At least 50w per plant of ACTUAL wattage, not equivalent. Get them 2-3 inches away if you want nice thick cola's.
Yep, if you have the buds in an airtight container and in a cool dark place, they will last up to 5 years. Potency will be lower, but still get you baked.
It will work for pH, but they arent the most accurate so i would be wary using it for ppm. Yees you you can use it in both soil and water but i found they werent good for reading the soil pH. Test runoff water instead.